A little advice required for a newby to mixing adhesive (previously only fitted small tiles with ready mix). I have 2 x 20kg bags of Keraflex Maxi S1 White - do I have to mix the whole bag as per instructions or can I mix part of the bag?

Hard to describe consistency really but you will see what I mean! Keraflex Maxi is a nice adhesive to work with.
What is the project you are using it for? Please feel free to share with us in another thread including photos!